1. Field of the Invention
The present invention relates to methods of preparing a compound semiconductor crystal and to the compound semiconductor crystals prepared thereby, and particularly to methods of preparing a carbon-containing, compound semiconductor crystal and compound semiconductor crystals obtained thereby.
2. Description of the Background Art
It has been conventionally well known that as for an LEC method using a stainless chamber there is a correlation between the CO gas concentration provided in the chamber and the carbon concentration of a GaAs crystal in a high-pressure Ar gas ambient.
FIG. 3 is a graph of carbon concentration in a GaAs crystal versus CO gas concentration in a LEC furnace found in Advanced Electronics Series I 4 Bulk Crystal Growth Technology, Keigo Hoshikawa, BAIFUKAN, p.184, Fig. 7.22. FIG. 3 shows that carbon concentration in a GaAs crystal and CO gas content in the LEC furnace are correlated by a straight line. In the LEC method, the correlation represented in the graph is applied to the adjustment of carbon concentration in a GaAs crystal. The carbon concentration in a GaAs crystal can be controlled by adjusting the CO gas content in the ambient gas using a CO gas cylinder and an Ar gas cylinder for dilution connected to the stainless chamber.
FIG. 4 shows an exemplary GaAs crystal growth equipment for the LEC method disclosed in Japanese Patent Laying-Open No. 1-239089. Referring to FIG. 4, Japanese Patent Laying-Open No. 1-239089 discloses a method of preparing a single crystal of compound semiconductor by placing in a predetermined gas ambient a raw-material housing portion housing a raw-material melt, detecting at least the concentration of one of H2, O2, CO2 and CO corresponding to components of the ambient gas, and controlling the detected concentration of a component at a predetermined value to keep over the entirety of an ingot a predetermined concentration of a residual impurity mixed into a resulting single crystal.
This method can, however, not be applied in preparing a compound semiconductor crystal in a gas-impermeable airtight vessel incapable of supplying a gas from outside the airtight vessel, such as a quartz ampoule.
Japanese Patent Laying-Open No. 3-122097 discloses a method of preparing a GaAs crystal in a quartz ampoule wherein a carbon source is arranged internal to the ampoule and external to a crucible in fluid communication with a polycrystalline compound provided as a raw material to allow the GaAs crystal to be doped with carbon. xe2x80x9cFluid communicationxe2x80x9d means a free flow of vapor and heat between the inside and outside of the crucible which allows carbon to be transferred into the crucible and thus to a melt. In accordance with the method, a carbon disk is arranged on an opening of a cap. It discloses that the ingots of various doped levels can be provided by varying the amount of carbon arranged external to the opening and/or the crucible.
With this method, however, a large amount of carbon source is placed above the melt. Thus fine powder of carbon falls thereon and can thus vary the carbon concentration thereof. Particularly, the controllability can be poor at a slight carbon concentration corresponding to a level of 0.1xc3x971015 cmxe2x88x923 to 2xc3x971015 cmxe2x88x923.
Japanese Patent Laying-Open No. 64-79087 discloses a method of preparing a single crystal of GaAs doped with carbon to reduce dislocation, using a reactor or a boat for crystal growth at least partially formed of carbon. It discloses that when a graphite boat is used, a part of the carbon boat changes into a gas (CO or CO2) due to oxygen derived from a small amount of As2O3, Ga2O or the like remaining in the quartz reactor and the gas is thus added to the single crystal of GaAs in synthesis reaction or in single-crystal growth.
In accordance with this method, however, it is difficult to control the carbon concentration in the crystal due to the difficulty of controlling the amount of As2O3, Ga2O or the like remaining in the quartz reactor. In particular, the controllability can be poor at a slight carbon concentration corresponding to a level of 0.1xc3x971015 cmxe2x88x923 to 2xc3x971015 cmxe2x88x923.
Japanese Patent Laying-Open No. 2-48496 discloses a method of preparing a Cr-doped, semi-insulating GaAs crystal wherein a quartz boat or a quartz crucible is used to grow the crystal under the existence of nitrogen oxide or carbon oxide. It discloses that when a GaAs crystal is grown under the existence of nitrogen oxide or carbon oxide, the oxide serves as an oxygen doping source to reduce the Si concentration of the grown crystal so that a semi-insulating crystal is reliably provided.
However, this method contemplates control of oxygen concentration and does not describe control of carbon concentration.
One object of the present invention is to provide a method of preparing a compound semiconductor crystal allowing the compound semiconductor crystal to be doped with carbon in high reproducibility, and a compound semiconductor crystal prepared thereby.
In one aspect of the present invention, a method of preparing a compound semiconductor crystal includes the steps of sealing carbon oxide gas of a predetermined partial pressure and a compound semiconductor provided as a raw material in a gas-impermeable airtight vessel, increasing the temperature of the airtight vessel to melt the compound semiconductor material sealed in the airtight vessel, and thereafter decreasing the temperature of the airtight vessel to solidify the melted compound semiconductor material to grow a compound semiconductor crystal containing a predetermined amount of carbon.
The carbon oxide gas includes at least one type of gas selected from the group consisting of CO gas and CO2 gas.
In growing the crystal, preferably the melted compound semiconductor material is at least partially kept in contact with boron oxide (B2O3).
In growing the crystal, more preferably the melted compound semiconductor material has its upper surface entirely covered with boron oxide (B2O3).
Preferably, the boron oxide (B2O3) has a water content of no more than 300 ppm, more preferably no more than 100 ppm.
Preferably, variation of the water content of the boron oxide (B2O3) is controlled to fall within a range from +20% to xe2x88x9220%.
In accordance with the present invention, the carbon oxide gas sealed in the airtight vessel preferably has a partial pressure of 0.1 to 100 Torr at 25xc2x0 C.
In accordance with the present invention, carbon oxide gas is preferably sealed in an airtight vessel according to an expression:
CCARBON=axc3x97P0.5xe2x80x83xe2x80x83(1),
wherein CCARBON (cmxe2x88x923) represents carbon concentration in a compound semiconductor crystal, P (Torr) represents partial pressure of the carbon oxide gas, and a represents any coefficient.
In expression (1) coefficient a preferably ranges from 0.25xc3x971015 to 4xc3x971015 cmxe2x88x923/Torr0.5, more preferably 0.5xc3x971015 to 2xc3x971015 cmxe2x88x923/Torr0.5.
In accordance with the present invention, preferably the step of subjecting the airtight vessel to a vacuum heat treatment is also provided before the step of sealing carbon oxide gas in the airtight vessel.
The vacuum heat treatment is preferably provided at a temperature of no more than 350xc2x0 C.
In accordance with the present invention, at least the internal wall of the airtight vessel and at least the outer surface of the contents of the airtight vessel other than the compound semiconductor material and the boron oxide are preferably formed from a material which does not contain carbon.
The material which does not contain carbon includes at least one material selected from the group consisting, e.g., of quartz, silicon nitride, boron nitride, pyrolytic boron nitride and alumina.
In accordance with the present invention, the gas-impermeable airtight vessel can at least partially be formed from quartz.
Preferably, the portion formed from quartz has a thickness of no less than 1.5 mm.
In growing the crystal, preferably the portion formed from quartz is controlled to have a temperature of at most 1270xc2x0 C.
In accordance with the present invention, in growing the crystal a space behind a raw-material melt of melted compound preferably has its most heated portion and its least heated portion with a temperature difference of no more than 300xc2x0 C. therebetween.
In accordance with the present invention, the space behind the raw-material melt is preferably larger, more preferably no less than twice larger in volume than the space on the side of the raw-material melt.
A method of preparing a compound semiconductor crystal in accordance with the present invention is applicable to preparing a compound semiconductor crystal of GaAs.
In another aspect, the present invention provides a compound semiconductor crystal prepared in accordance with the above-described method of preparing a compound semiconductor crystal, having a carbon concentration of 0.1xc3x971015 cmxe2x88x923 to 20xc3x971015 cmxe2x88x923.
In accordance with the present invention, the compound semiconductor includes GaAs.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.